Sintered metal parts and method for the manufacturing thereof

ABSTRACT

The invention relates to a method consisting of the steps of providing a pre-alloyed iron-based metal powder comprising at least 1.3-3.5% chromium, 0.15-0.7% molybdenum, manganese and unavoidable impurities, mixing said powder with 0.1-1.0% graphite, compacting the obtained mixture at a pressure of at least 600 MPa, sintering the compacted part in a single step at a temperature above 1100° C., shot-peening the part and after sintering optionally hardening the component. The invention also relates to a powder metallurgical part and use of a low chromium prealloyed powder for preparing notched sintered parts having a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm 3 , preferably at least 400 MPa at a sintered density of 7.3 g/cm 3 .

FIELD OF THE INVENTION

The present invention refers to powder metallurgy and more specifically to pre-alloyed chromium powder metal parts with improved fatigue properties.

BACKGROUND OF THE INVENTION

In general, sintered products made by powder metallurgy are advantageous in cost over ingot steels obtained through forging and rolling steps and has wide utility as parts in e.g. motor vehicles. However, the sintered product has pores which are inevitably formed during the course of its fabrication. These remaining pores of the sintered powder-metallurgical materials impair the mechanical properties of the materials, as compared with completely dense materials. This is a result of the pores acting as stress concentrations and also because the pores reduce the effective volume under stress. Thus, strength, ductility, fatigue strength, macro-hardness etc. in iron-based powder-metallurgical materials decrease as the porosity increases.

Despite their comparatively low fatigue strength , iron-based powder-metallurgical materials are, to a certain extent, used in components requiring high fatigue strength. Distaloy® HP, available from Höganäs AB®, Sweden, is a steel powder possible for use in high performing purposes. In this Distaloy®-powder the base-powder is alloyed with nickel, which is an expensive alloying element. This high performing material is therefore rather costly and there is a need for less expensive materials, which have at least as good fatigue strength.

One route to improve the fatigue performance of powder metallurgical steels are secondary operations. Through hardening, case hardening or shot peening (or a combination) are possible processes to get highest possible fatigue resistance of a component. Shot peening is normally performed in order to utilize the beneficial influence of compressive residual stresses in the surface. Pores open to the surface are weak points in powder metallurgical materials. These pores are at least partly neutralized by introduction of surface compression residual stresses.

Shot peening of compacted parts is disclosed in e.g. the U.S. Pat. No. 6,171,546. According to this patent the shot peening is followed by a final sintering step. An iron-based powder containing i. a. nickel is used as starting material. As indicated above there is an increasing demand for powders, which do not contain nickel, as nickel is expensive. Other disadvantages with nickel containing powders are dusting problems which may occur during the processing of the powder, and which may cause allergic reactions also in minor amounts. The use of nickel should thus be avoided. Also the U.S. patent application 2004/0177719 relates to a method including shot peening, More specifically, this application discloses a method, wherein a portion of the surface of a compacted part is subjected to shot peening after sintering. According to this application a densifying process involving powder forging or sizing is necessary in order improve the properties of the final compacted part.

An object of the present invention is to provide a cost effective process for the preparation of powder metallurgical components with high fatigue strength without any steps for achieving core densification. Another object is to provide a process involving powder materials, which are free from nickel.

SUMMARY OF THE INVENTION

It has unexpectedly been found that components having high fatigue strength can be obtained by shot peening of sintered components prepared from iron based powders distinguished by low levels of chromium and molybdenum.

DETAILED DESCRIPTION OF THE INVENTION

The powders used in the present invention are pre-alloyed iron-base powders comprising low amounts of chromium and molybdenum. A preferred amount is 1.3-3.5% by weight of chromium and 0.15-0.7% by weight of molybdenum. The powder may also contain small amounts, 0.09 to 0.3% by weight, of manganese and inevitable impurities. Such powders are known from the U.S. Pat. No. 6,348,080 and WO 03/106079.

The base powder is further mixed with graphite to obtain the desired strength in the material. The amount of graphite which is mixed with the iron-base powder is 0.1-1.0%, preferably 0.15-0.85%. The powder mixture is compacted in a die to produce a green body. The compaction pressure is at least 600 MPa, preferably at least 700 MPa and more preferably 800 MPa. The compaction can be performed by cold compaction or warm compaction. After the compaction the obtained green part is sintered at a sintering temperature above 1100° C., preferably above 1220° C. The sintering atmosphere is preferably a mix of nitrogen and hydrogen. A normal cooling rate in the sintering process is 0.8° C./s, a range between 0.5° C./s and 1.0° C./s is preferred. The sintered density is preferably above 7.15 g/cm³, more preferably above 7.3 g/cm³. The obtained microstructure in the as-sinterd material is mainly fine-pearlitic with a lower chromium and molybdenum content and martensitic or lower bainitic for slightly higher chromium and molybdenum content.

It has now unexpectedly been found that a remarkable increase in the bending fatigue limit can be obtained by shot peening the sintered low chromium powder materials. Especially remarkable increase is obtained for notched parts, where an increase of more than 50% and even more than 70% can be obtained as can be seen from the following examples. The degree of shot peening as defined by Almen A intensity, is preferably between 0.20 and 0.37 mm.

Secondary operations e.g. through hardening and case hardening, can be performed before the shot peening in order to improve the properties even more. Thus, after through hardening followed by tempering the material is mainly martensitic and the fatigue limit is raised by shotpeening. The martensite in the surface which is formed during case hardening is believed to form compressive stresses, which is beneficial for the fatigue limit.

Sinterhardening is an alternative process which is applied in the sintering process. Sinterhardening uses forced cooling at the end of the sintering process of the components which results in a hardened structure.

The fatigue tests have been performed on notched specimen with a stress concentration factor, K_(t), of 1.38 and on un-notched specimen. The tests show a greater increase in bending fatigue limit when shot peening notched specimen than when the shot peening is performed on un-notched specimen. The expression “notched” in this context refers to a specimen or component having a stress concentration factor above 1.3.

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1

Two pre-alloyed base-powders, Astaloy® CrL and Astaloy® CrM, and one diffusion-alloyed base powder, Distaloy® HP, are included in the study. Distaloy® HP is diffusion-alloyed with Ni and Cu and pre-alloyed with Mo. The three materials included in this study are shown in Table 1. TABLE 1 Material Ni [%] Cu [%] Mo [%] Cr [%] Astaloy CrL 0.2 1.5 Astaloy CrM 0.5 3.0 Distaloy HP 4.0 2.0 1.5

Detailed information on process parameters, density and carbon levels will be given below. In table 2 plane bending fatigue performance of un-notched specimen is shown for different alloys which are sintered 30 min in 90/10 N₂/H₂ with cooling rate about 0.8 C/sec. Fatigue tests on un-notched specimens are performed using 5 mm ISO3928 samples with chamfered edges. The tests are made in four-point plane bending at load ratio F=−1. The staircase method is used with 13-18 samples in the staircase and 2 million cycles as run-out limit. Evaluation of the staircase (50% probability fatigue limit and standard deviation) is made according to the MPIF 56 standard. Test frequency is 27-30 Hz. TABLE 2 Density Carbon σ_(A, 50%) Std Dev σ_(A, 90) Powder [g/cm³] As-Sint [%] [MPa] [MPa] [MPa] Astaloy CrL 7.17 0.60 244 7 234 7.16 0.80 267 5 260 Astaloy CrM 7.06 0.35 284 7.0 274 7.04 0.56 316 8.4 300 Distaloy HP 7.13 0.65 295 22.5 261 7.13 0.85 330 <5 >322

The microstructure of Astaloy CrL with sintered carbon below 0.6% and cooling rate about 0.8° C./s is upper bainite. Increased carbon above 0.74% changes the microstructure to fine pearlite.

Microstructure analysis of 1120° C. sintered Astaloy CrM materials and cooling rate 0.8° C./s and with sintered carbon levels between 0.32% and 0.49% show a dense upper bairitic microstructure. Dense upper bainite has the same characteristics as regular upper bainite, i.e. an irregular mix of ferrite and cementite. The differences are the smaller distances between carbides and sizes of the carbides. Increased sintered carbon shifts the microstructure to a mix of martensite and lower bainite.

Table 3 shows influence of compaction pressure and carbon level for cold compacted Astaloy CrL. All materials were sintered at 1120° C. for 30 min. in 90/10 N₂/H₂. In table 3 a summary of plane bending fatigue performance of Astaloy CrL at two compaction pressures and two levels of additional graphite. Std.dev. <5 indicates that the scatter is small and the MPIF standard 56 evaluation of standard deviation cannot be applied. The specimen in table 3 are un-notched. TABLE 3 Carbon Graphite Compacting Density As- Std C-UF4 Pressure As-sint sint σ_(A, 50%) Dev σ_(A, 90) Material [%] [MPa] [g/cm³] [%] [MPa] [MPa] [MPa] Astaloy CrL 0.6 600 6.94 0.56 224 11.6 205 1120° C., 0.8 600 6.93 0.75 233 9.5 218 30 min, 0.6 800 7.13 0.55 236 8.5 222 90/10 N₂/H₂ 0.8 800 7.09 0.74 252 <5 >244 0.8° C./5

Influence of sintering temperature on the fatigue performance with un-notched specimen is shown in Table 4. The microstructures of the materials in table 4 are characterized by mainly upper bainite (1120° C. 0.58-% C) and fine pearlite (1120° C., 0.77% C and 1250° C., 0.74% C). TABLE 4 Density Carbon Sint. As-Sint As-Sint σ_(A, 50%) StdDev σ_(A, 90) Powder temp [g/cm³] [%] [MPa] [MPa] [MPa] Astaloy 1120° C. 7.10 0.58 220 11 203 CrL 1120° C. 7.08 0.77 236 9.7 222 1250° C. 7.02 0.74 290 18 264

EXAMPLE 2

Influence of shot peening and the combination of heat treatment and shot peening has been investigated on Astaloy CrL 3 mm edge-notched specimens. The notch is included in the press tool and no machining is performed. The stress concentration factor in bending is obtained by FEM to K_(t)=1.38. Test frequency is 27-30 Hz.

The materials are sintered at 1280° C. for 30 min in H₂. Cooling rate is 0.8° C./s.

The shot peening is performed to obtain an Almen A intensity of 0.32 nm.

Estimated plane bending fatigue performance of as sintered and as-sintered plus shot peened samples is shown in table 5. TABLE 5 Bending Carbon Density Secondary Fatigue Increase As-Sint As-Sint operation Limit after shot Powder [%] [g/cm³] Shot peening [MPa] peening Astaloy CrL 0.70 7.30 NO 235 notched YES 420 +79% Astaloy CrL 0.85 7.30 NO 340 un-notched YES 450 +32%

In table 6 an estimated plane bending fatigue performance of through hardened tempered and shot peened samples is shown. Through hardening is performed with an austenitization temperature at 880° C. The cooling rate after austenitization is made at 8° C./s. Finally the specimen are tempered at 250° C. for 1 hour TABLE 6 Carbon Bending As- Density Secondary operations Fatigue Increase Sint As-Sint Through Tempering Shot Limit after shot Powder [%] [g/cm³] hardening 250° C. 1 h peening [MPa] peening Astaloy CrL 0.50 7.30 YES YES NO 285 notch 0.50 7.30 YES YES YES 490 +73% Astaloy CrL 0.50 7.30 YES YES NO 370 un-notch 0.50 7.30 YES YES YES 520 +41%

From the tables 5 and 6 it can be found that by shot peening the materials containing chromium and molybdenum a great increase of the bending fatigue limit is achieved. 

1. A method for producing powder metallurgical parts with improved fatigue strength comprising the steps of: providing a pre-alloyed iron-based metal powder comprising at least 1.3-3.5% by weight of chromium, 0.15-0.7% by weight of molybdenum, mixing said powder with 0.1-1.0% by weight of graphite, compacting the obtained mixture at a pressure of at least 600 MPa, sintering the compacted part in a single step at a temperature above 1100° C., and shot-peening the part.
 2. A method according to claim 1 wherein the increase of the fatigue strength is at least 50%.
 3. The method according to claim 1, wherein the compacted and sintered part is subjected to hardening and tempering prior to shot peening.
 4. A powder metallurgical part manufactured according to claim 1 having a mainly pearlitic microstructure.
 5. A powder metallurgical part manufactured according to claim 1 having a martensitic and lower bainitic microstructure.
 6. The powder metallurgical part manufactured according to claim 1 having a mainly tempered martensitic microstructure.
 7. The powder metallurgical part according to claim 1 having a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm³.
 8. A method for preparing notched sintered parts having a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm³, comprising compacting a low chromium Prealloyed Powder to form a compacted part, sintering and optionally tempering and annealing said compacted part, and thereafter subjecting said compacted part to shot peening.
 9. A method for preparing notched sintered parts according to claim 8, wherein said parts have a stress concentration factor above 1.3.
 10. The method according to claim 2, wherein the compacted and sintered part is subjected to hardening and tempering prior to shot peening.
 11. A powder metallurgical part manufactured according to claim 2 having a mainly pearlitic microstructure preferably mainly pearlitic microstructure.
 12. A powder metallurgical part manufactured according to claim 3 having a mainly pearlitic microstructure preferably mainly pearlitic microstructure.
 13. A powder metallurgical part manufactured according to claim 1 having a mainly fine pearlitic microstructure.
 14. A powder metallurgical part manufactured according to claim 2 having a martensitic and lower bainitic microstructure.
 15. A powder metallurgical part manufactured according to claim 3 having a martensitic and lower bainitic microstructure.
 16. The powder metallurgical part manufactured according to claim 2 having a mainly tempered martensitic microstructure.
 17. The powder metallurgical part manufactured according to claim 3 having a mainly tempered martensitic microstructure.
 18. A powder metallurgical part according to claim 1 having a bending fatigue limit of at least 400 MPa at a sintered density of 7.3 g/cm³.
 19. The powder metallurgical part according to claim 2 having a bending fatigue limit of at least 340 MPa at a sintered density of 7.15 g/cm³.
 20. A method for preparing notched sintered parts according to claim 8 wherein said parts have a bending fatigue limit of at least 400 MPa at a sintered density of 7.3 g/cm³. 